1. Field of the Invention
This invention is related to an optical fiber drop cable which is used when optical fibers are dropped from an optical fiber cable to houses or the like.
2. Description of Relevant Art
Up until now, for purposes of FTTH (Fiber to the Home), in order to transmit and receive high speed broadband information such as very high speed data at home or the office, the optical fiber must be pulled from the optical fiber cable of the access system, which is extending from the telephone office, into subscribers' homes such as buildings or general houses. The optical fiber drop cable is used as the optical fiber cable which is preferable for this wiring. That is, the optical fiber drop cable (outdoor fiber cable) is a cable which is used when bringing in optical fibers from a telephone pole into a house.
With reference to FIG. 1, the optical fiber drop cable 101 comprises an optical element section 109 and a cable support section 115. The optical element section 109 includes an optical fiber tape core wire 103 (or optical fiber core wire), which consists of four core optical fibers, a pair of tensile strength bodies 105 disposed on both sides of the optical fiber tape core wire 103 in parallel, which consist of electroconductivity metal wire, for example, 0.4 mm diameter steel wires, and a cable sheath 107 covering them, which is made of thermoplastic resin such as PVC and flame-retardant PE. The cable support section 115 includes a support wire 111 consisting of a metal wire such as a steel wire 1.2 mm in diameter, and a sheath 113 covering the support wire 111, which is made of thermoplastic resin such as PVC and flame-retardant PE. The optical element section 109 and the cable support section 115 are parallel to each other and connected as one body via a narrowed neck portion 117. In the cable sheath 107, a notch 119 is provided on each side of the upper and lower direction (y direction) in FIG. 1 to take out the tape core wire 103.
With reference to FIGS. 2A, 2B and 2C, the shape of the neck portion 117 is as shown in the figure, and the width (x direction size) of the neck portion 117 is 0.2-0.4 mm. In FIG. 2A, the neck portion 117 has flat sides which are parallel to the x direction, and the thickness (y direction size) of the neck portion 117 is 0.15-0.35 mm. In FIG. 2B, the neck portion 117 has a constricted shape, where the joint thickness of the cable support section 115 side and the optical element section 109 side is 0.7-0.8 mm and the thickness about the central portion is 0.15-0.35 mm. In FIG. 2C, the joint thickness at the cable support section 115 side is 0.7-0.8 mm and is shaped slanted towards the joint thickness 0.15-0.35 mm of the optical element 109 side.
With reference to FIG. 3 which shows the production line for producing optical fiber drop cable 101, wherein the tape core wire 103, tensile strength bodies 105, and support wire 111 are each supplied by bobbins 121, 123, 125 and sent to an extrusion head 131 of an extrusion machine 129 via a branch line board 127. On both sides of the tape core wire 103, the tensile strength bodies 105 are placed in parallel and supplied to the prescribed position in the extrusion head 131. The support wire 111 is also supplied to the prescribed position in the extrusion head 131.
The thermoplastic resin used commonly as the cable sheath 107, 113 and the neck portion 117 is extruded into the extrusion head 131 for extrusion covering together. That is, at the same time when the tape core wire 103 and the tensile strength bodies 105 are covered with the cable sheath 107, and the support wire 111 is covered with the sheath 113 inside the extrusion head 131, the cable sheath 107 and sheath 113 are connected as one body by the neck portion 117.
With referring to FIG. 4, so that the tape core wire 103 of the optical fiber drop cable 101 is pulled from the optical fiber cable 133 extending from the telephone office, into each building or house, the optical element section 109 and the cable support section 115 are separated by partially splitting the neck portion 117 of both ends of the optical fiber drop cable 101. One end 115A of the cable support section 115 is fastened to a telephone pole 135 by an outdoor clamp 137, and the other end 115B is fastened to a part of the house by the clamp 137.
By use of the notch 119 of the optical element section 109, the cable sheath 107 of one end 109A of said optical element section 109 is cut away, and the tape core wire 103 is pulled out to be connected to a cable branching box (cable closure) 139 on the telephone pole 135. For the other end 109B, the cable sheath 107 is cut away by use of the notch 119 of the optical element 109, and the tape core wire 103 is pulled out to be connected to an OE converter or termination box 141 inside the house.
Now, in case of said optical fiber drop cable 101 or other conventional optical fiber drop cable has the size and shape of neck portion 117 which are the same as shown in FIGS. 2A-2C, there was a problem of adherence of the optical element section 109 and the cable support section 115 in manufacturing the cable 101, which is difficult to separate. This posed a problem in that it diminished the quality of work when laying a cable.
There was also the problem of the exposure of the support wire 111 when the optical element section 109 and the cable support section 115 were separated.